Combustion of hydrocarbon-based fuel in electrical power stations and in engines produces exhaust gas that contains, in large part, relatively benign nitrogen (N2), water vapour (H2O), and carbon dioxide (CO2). But the exhaust gases also contain, in relatively small part, noxious and/or toxic substances, such as carbon monoxide (CO) from incomplete combustion, hydrocarbons (HC) from un-burnt fuel, nitrogen oxides (NOx) from excessive combustion temperatures, and particulate matter (mostly soot). To mitigate the environmental impact of exhaust gas released into the atmosphere, it is desirable to eliminate or reduce the amount of the undesirable components, preferably by a process that, in turn, does not generate other noxious or toxic substances.
Typically, exhaust gases from electrical power stations and lean burn engines have a net oxidizing effect due to the high proportion of oxygen that is provided to ensure adequate combustion of the hydrocarbon fuel. In such gases, one of the most burdensome components to remove is NOx, which includes nitric oxide (NO) and nitrogen dioxide (NO2). The reduction of NOx to N2 is particularly problematic because the exhaust gas contains enough oxygen to favour oxidative reactions rather than reduction. Notwithstanding, NOx can be reduced by a process commonly known as Selective Catalytic Reduction (SCR). The SCR process involves the conversion of NOx, in the presence of a catalyst and with the aid of a nitrogenous reducing agent, such as ammonia, into elemental nitrogen (N2) and water. In a SCR process, a gaseous reductant such as ammonia is added to an exhaust gas stream prior to contacting the exhaust gas with the SCR catalyst. The reductant is absorbed onto the catalyst and the NO reduction reaction takes place as the gases pass through or over the catalysed substrate. The chemical equations for SCR with ammonia are:4NO+4NH3+O2→4N2+6H2O2NO2+4NH3+O2→3N2+6H2ONO+NO2+2NH3→2N2+3H2O
Some ammonia may pass through the SCR catalyst without reacting (also referred to as “ammonia slip”) and this is undesirable, because the released ammonia gas can negatively impact the atmosphere and can react with other combustion species. To reduce ammonia slip, SCR systems can include an ammonia oxidation catalyst (AMOX) (also known as an ammonia slip catalyst (ASC)) downstream of the SCR catalyst.
Catalysts for oxidizing excess ammonia in an exhaust gas are known. For example, U.S. Pat. No. 7,393,511 describes an ammonia oxidation catalyst containing a precious metal, such as platinum, palladium, rhodium, or gold on a support of titania alumina, silica, zirconia, etc. Other ammonia oxidation catalysts contain a first layer of vanadium oxide, tungsten oxide, and molybdenum oxide on a titania support, and a second layer of platinum on a titania support (see, e.g., U.S. Pat. Nos. 8,202,481 and 7,410,626).
Current ASC technology uses a combination of an oxidation function and a SCR function to selectively convert NH3 to N2. In gas turbine applications, the primary function of an ASC is to control NH3 slip. However, as an ASC has an oxidation function, the ASC could also replace upstream oxidation catalysts that are used to oxidize volatile organic carbons (VOC's) and carbon monoxide (CO) resulting in a SCR-ASC catalyst system which is simpler and potentially lower cost.
Due to stricter emissions requirements, gas turbines are now typically being required to emit less than 5 ppm CO, which can require 90% or higher CO conversion. The CO conversion provided by the ASC may not be sufficient to meet the required system performance targets. Thus in many cases it is desirable to add an additional CO oxidation function to the SCR-ASC system. However since the ASC is already catalysing significant CO oxidation, the additional CO oxidation capability required is small. This system would have a (using an upstream to downstream notation) SCR-ASC-CO Oxidation catalyst configuration.
Since only a relatively small amount of CO oxidation capability may be required in the SCR-ASC-CO Oxidation catalyst system, the CO oxidation catalyst may have a relatively small depth. This can present a problem in the manufacturing and packaging of the catalyst. It is the purpose of this invention to address this manufacturing and packaging problem.